In my experience, this issue is related to mclk switching and it affects the
lowest mclk level only.

So you guy's can save a lot of power, if you, insteed of switching to highest
gfxlevel or to disable vsync, just disable the lowest mclk level by:

echo "manual" > /sys/class/drm/card0/device/power_dpm_force_performance_level
echo "1 2 3" > /sys/class/drm/card0/device/pp_dpm_mclk

If you are building your kernel locally, look in this thread for a driver code
modification that works, without disabling the lowest mclk level (saves a few
watt on idle).